Epoxy resins have been traditionally used to a large extent as matrix resins for composite materials due to their adhesiveness or high rigidity. However, as the performance level required for the composite materials is ever increasing, various investigations on the matrix resins being used are in progress. The performances required for the composite materials include, for example, heat resistance, impact resistance (toughness), and mechanical properties under high temperature and high humidity conditions (particularly interlayer shear strength (ILSS), or the like).
Specifically, an epoxy resin composition comprising N,N,N′,N′-tetraglycidylmethane (TGDDM) as a main component, and 4,4′-diaminodiphenylmethane (DDS) as a curing agent, has been used extensively in applications where heat resistance is required. However, although this composition has excellent heat resistance, rigidity, or the like, it has low impact resistance. Accordingly, in order to confer impact resistance, a bifunctional epoxy resin, e.g., a bisphenol A type epoxy resin, may be used as a main component. However, in this case, heat resistance is lowered, thereby required performances often becoming unsatisfactory. Furthermore, as an attempt to improve impact resistance, there has been proposed a method of adding a rubber-like polymer such as an acrylonitrile-butadiene copolymer to an epoxy resin, etc., and then micro-phase separating a rubber layer comprising the rubber-like polymer during the curing of the epoxy resin. However, this method also has a tendency to lower heat resistance or rigidity being lowered.
As such, it has been very difficult to attain satisfaction both in impact resistance and heat resistance.
Furthermore, as disclosed in Patent Documents 1 and 2, for example, there is a method of adding a thermoplastic resin, e.g., polyethersulfone (PES), to an epoxy resin composition having high heat resistance, in order to confer impact resistance to the epoxy resin composition. However, in order to obtain a certain effect through this method, it is necessary to add a large amount of a thermoplastic resin, and as a result, the viscosity of the epoxy resin composition is increased. Generally, in the production of a golf club shaft or a tennis racket comprising a composite material, a prepreg obtained by impregnation of reinforcing fibers with an epoxy resin composition is used as a raw material, and this prepreg is usually prepared by heating and pressurizing reinforcing fibers which have been arranged in one direction over an epoxy resin composition applied on a silicone-coated paper, thus to impregnate the reinforcing fibers with the epoxy resin composition. For this reason, an increase in the viscosity of such an epoxy resin composition significantly deteriorates characteristics such as appropriate adhesion and flexibility that are highly required for a prepreg. Also, as the viscosity of the epoxy resin composition is increased, the capability of the prepreg to pass through the production process is also remarkably lowered.
As a method for improving impact resistance, there is proposed in Patent Document 3 a method for improving the interlayer peeling strength in a multi-layer composite material. This method involves distributing fine particles of a thermoplastic resin concentrated between the layers, but such a method cannot avoid significant reduction in the level of adhesion of a prepreg, while posing new problems such as complications in processes and complications in quality control. Patent Document 3 also discloses the use of fine particles of nylon 6 as the fine particles of thermoplastic resin; however, nylon 6 absorbs moisture under high temperature and high humidity conditions, usually to an extent of 4.5% by weight, and such moisture absorption may deteriorate the mechanical properties of the composite material.
Furthermore, for the same purpose, there has been proposed an attempt to localize chopped fibers or milled fibers of nylon or the like between the layers. However, this method cannot avoid the same problem concerning moisture absorption, and the effect is not necessarily satisfactory.
In addition, as a method for improving the impact resistance, there are proposed methods involving inserting between the layers a sort of impact absorbing layer which is referred to as an interleaf (see Patent Documents 4 to 7). However, in all of these methods, the interlayer thickness is increased, and thus the fiber ratio may be decreased, or the heat resistance, and mechanical strength under high temperature and high humidity conditions of the resulting composite material, the adhesion of the resulting prepreg, and the like may be deteriorated, thereby handlability being lowered, or the like.
Therefore, the present applicant has proposed in Patent Document 8 a technology relating to a prepreg that has heat resistance, impact resistance, and mechanical properties under a high temperature and high humidity, while maintaining appropriate properties required for a prepreg, such as adhesion and flexibility, or a good capability to pass through the production process, in which prepreg a resin composition obtained by blending a reaction mixture of a bifunctional epoxy resin, a trifunctional epoxy resin, and a phenol compound, with a tetrafunctional epoxy resin and an aromatic amine, is used as the matrix resin.
However, recently, the demand for higher performance of composite materials in the market has further increased, and a material is desired which simultaneously satisfies higher heat resistance, impact resistance, and mechanical properties under high temperature and high humidity.
Further, Patent Document 8 describes that by further blending an elastomer such as a butadiene-acrylonitrile copolymer having carboxylic groups on both ends, with the above-described resin composition, higher impact resistance can be exhibited. However, if a rubber component such as the elastomer is simply blended, the impact resistance may be improved in accordance with the blending amount, but the heat resistance is again deteriorated.
Furthermore, there is also proposed a method for inhibiting reduction in heat resistance as well as improving the mechanical properties under high temperature and high humidity conditions, by blending a heat resistant component. For example, Patent Documents 9 to 11 disclose resin compositions obtained by blending an epoxy resin with a polyamide resin. However, even if these resin compositions were used for prepregs, sufficient effects would not be obtained.
Moreover, Patent Documents 12 to 15, Patent Documents 16 to 17, and Patent Document 18 each describe that a specific polyamide resin is added to an epoxy resin, and this mixture is used as the matrix resin of a prepreg. However, if the resin composition as described in Patent Documents 12 to 15 is used in a prepreg, problems occur such as an insufficient improvement of impact resistance, poor capability of the prepreg to pass through the production process, and short work life of the prepreg. Further, with the technology as described in Patent Documents 16 to 17, the phase structure of the matrix resin varies depending on the curing conditions or the state of mixing of the matrix resin. As a result, it becomes difficult to simultaneously satisfy both the heat resistance and the mechanical properties under high temperature and high humidity. Also, in the technology of Patent Document 18, impact resistance may be improved, but there is a drawback that rigidity satisfying the requirements of the market cannot be ensured.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S58-124126
[Patent Document 2] Japanese Unexamined Patent Application Publication No. S62-153349
[Patent Document 3] Japanese Unexamined Patent Application Publication No. H1-110537
[Patent Document 4] U.S. patent application Ser. No. 3,472,730
[Patent Document 5] Japanese Unexamined Patent Application Publication No. S51-58484
[Patent Document 6] Japanese Unexamined Patent Application Publication No. S60-63229
[Patent Document 7] Japanese Unexamined Patent Application Publication No. S60-231738
[Patent Document 8] Japanese Patent No. 3026372
[Patent Document 9] Japanese Examined Patent Publication No. S40-1874
[Patent Document 10] Japanese Unexamined Patent Application Publication No. S55-71771
[Patent Document 11] Japanese Unexamined Patent Application Publication No. S56-152832
[Patent Document 12] U.S. patent application Ser. No. 2,705,223
[Patent Document 13] U.S. patent application Ser. No. 2,986,539
[Patent Document 14] Japanese Unexamined Patent Application Publication No. S58-53913
[Patent Document 15] Japanese Unexamined Patent Application Publication No. S63-99222
[Patent Document 16] Japanese Unexamined Patent Application Publication No. S61-103992
[Patent Document 17] Japanese Unexamined Patent Application Publication No. S64-6019
[Patent Document 18] Japanese Unexamined Patent Application Publication No. H3-203923